Page:Natural History, Mollusca.djvu/97

their structure. They inhabit lakes, ponds, and ditches of fresh water; are found, but less commonly, in rivers, and still more rarely in brackish water. They crawl on the mud at the bottom, or on the stems and leaves of aquatic plants, always coming to the surface, at intervals, to take in a fresh supply of air into the lung chamber for respiration. They may frequently be seen floating, at the surface of still water, by the expanded foot, the shell being downward and submerged.

They lay their eggs in round or oval masses of consistent jelly, each mass containing a number of eggs, varying from three to upwards of a hundred, according to the genus. The masses are attached to plants or stones beneath the surface, and are hatched in about a fortnight after they are deposited.

The Pond-snails are very numerous, and widely distributed, species being found in almost all parts of the world. Twenty-four are reckoned by Messrs. Forbes and Hanley as British.

"It had been supposed that the shells of fluviatile Mollusca could be distinguished from those of the terrestrial kinds, by the edge of the mouth of the shell never being furnished with a thickened internal rib, and not being in the slightest degree reflexed, and that the animal never closes it with an epiphragm; however, further examination has shown that when the Pond-snails and the Whirl-shells are left nearly dry by the evaporation of the water, either by the heat, or by dryness of the weather in winter, these animals assume the character of terrestrial Mollusca, thicken and reflect their mouth, and form an epiphragm to prevent themselves from being destroyed by the drying up